Regarding an oxide thin film transistor (Oxide TFT), in order to improve stability, a dual-gate (namely, two gate electrodes) structure is applied in the TFT. As shown in FIG. 1, two gate electrodes, namely, a top gate electrode 1 at the top and a bottom gate electrode 2 at the bottom, are included in a TFT structure, and a gate insulating layer and an active layer that is formed of an oxide are located between the top gate electrode 1 and the bottom gate electrode 2. The top gate electrode 1 and the bottom gate electrode 2 act jointly to turn on the TFT. The dual-gate structure has the following advantages over a single-gate structure: channels are formed on upper and lower sides, respectively, so that an on-state current (Ion) is increased, and accordingly, it is possible that the size of the TFT is decreased, and the transmittance is enhanced; furthermore, when an oxide semiconductor layer is adopted, the gate metal can provide a function of shielding protection of the oxide on two sides of the semiconductor layer, so as to prevent occurrence of a leakage-current phenomenon owing to incidence of an external light onto the semiconductor layer, and in turn, the possibility that undesired phenomena such as afterimages and so on occur is reduced.